The present invention relates to spinal implants for osteosynthesis devices and, more particularly, to a spinal implant, a driver tool specifically suited for the spinal implant, and a nut guide suited for the spinal implant.
In recent years, various research and development have been attempted to provide a spinal implant that is designed to achieve adaptation to differences in alignment, augulation and depth of penetration of adjacent spinal implants anchored to vertebral bodies which are spaced from one another.
One of such implants is disclosed in U.S. Pat. No. 5,154,719. This device includes a head portion having a pair of upright branches which are internally threaded and which has a U-shaped recess, and a screw portion projecting from the head portion. A connecting rod is received in the two branches, which are fixed in place by means of a ring member. During this operating step, there exist some difficulties in precisely adjusting the orientation of the U-shaped recesses of the spaced spinal implants anchored in vertebral bodies, angulation of the spinal implants and depth of the spinal implants, with a resultant undesirable fixation of the implants and the connecting rod. Since, further, the screw portion is composed of a solid material and the screw portion is merely anchored in the vertebral body. Accordingly, after the spinal implant penetrates in the vertebral body, fixation of the spinal implant is unstable and is liable to be undesirably affected with external forces when they are applied to the spinal implants.
U.S. Pat. No. 5,879,351 discloses a spinal osteosynthesis device comprising at least one vertebral rod, pedicle screws and deformable connectors. In this prior art, each of the pedicle screws is composed of the same solid material as in the prior art discussed above, and a difficulty is similarly encountered in reliably fixing the pedicle screw in the vertebral body. Further, each of the deformable connectors has an oblong opening through which a head portion of the screw extends and each connector is resiliently supported between a cylindrical base and a nut, with a given space being provided for permitting relative movement of the vertebral rod. With such a structure, the spinal implant is caused to have a large number of component parts, resulting in a complicated structure and an increased cost.
In known techniques, it has been a usual practice to lock a plug into the spinal implant by means of a tool holder. In practice, there are two types of head section formed with two upright branches, that is, a first type of head section having an internally formed thread, and a second type of head section having an outwardly formed thread. In the first type, the plug is screwed in the internal thread of the head section. In this event, the two branches are loosened, thereby providing a difficulty in tightly locking the connecting rod in the spinal implant. On the contrary, in the second type, the plug is screwed onto the outer thread of the head section. In this event, an outer periphery of the plug has a hexagonal profile, and a driver tool having a hexagonal groove is brought into engagement with the outer hexagonal wall of the plug for rotating the plug. In this event, since the driver tool has an outer diameter larger than that of the plug, increasing an occupying space for rotating the driver tool. Under these conditions, when two spinal implants are anchored in adjacent vertebral bodies in a relationship closer to one another, the outer periphery of the driver tool is liable to interfere with the adjacent plug of the spinal implant, causing difficulties in rotating operation of the driver tool.